1. Field of the Invention
The present invention relates to novel siliceous particulates, and, more especially, to such particulates, advantageously beads, useful as silica pigments and reinforcing fillers for elastomeric matrices. The invention also features a novel process for the preparation of such readily dispersible siliceous particulates.
2. Description of the Prior Art
It has long been known to this art to attempt to substitute the white fillers/pigments, such as silica, for such conventional black fillers as carbon black.
However, such alternate fillers must first be capable of ready dispersion within a given elastomeric matrix. Furthermore, the elastomer thus reinforced has to be imbued, not only with good static mechanical properties, but also with the especial dynamic properties required for any given intended application thereof.
In the particular application of tire reinforcement, for example, such fillers must not give rise to internal heating when the tire is in dynamic use.
An attempt was first made to establish certain correlations between silica and its behavior in elastomers by analogy to the carbon black art. Thus, it was attempted to establish any relationship between the BET surface area and such behavior. But this approach produced results which not only were unsatisfactory, but even contradictory. Indeed, any such correlations, which sometimes appear to be incorrect, seem to be at least partially due to the differences in morphology between the carbon blacks and the silicas.
In particular, the carbon blacks which are used as reinforcing fillers typically have a high primary structure and a weak secondary structure, whereas the secondary structure of the precipitated silicas is far from being negligible as compared to their primary structure. The primary structure is understood to be the result of coalescence of the ultimate particles which form the aggregates. It will of course be appreciated that the bonds between ultimate particles in an aggregate are strong chemical bonds.
The secondary structure is known to be the result of physical interactions between the aggregates, to form agglomerates. In contrast to the case of the primary structure, these bonds are weak and are physical in nature. Moreover, it too has been postulated that the primary structure of silica is different from that of the carbon blacks.
Thus, this art is replete with attempts to alter the morphology of precipitated silicas in order to improve their behavior/properties. Compare, for example, published French patent application No. 76/16962 which features a silica having a CTAB surface area ranging from 80 to 125 m.sup.2 /g and a structural index of at least 0.80.
Also, a silica in powder form has the known disadvantage of forming dust when handled. It was therefore ascertained to be preferred to granulate the silica.
With such granulated silica it was determined, however, (a) that the degree or extent of dispersion was less with granulated than with non-granulated silica, and (b) that the degree of reinforcement therewith was also less, as can easily be demonstrated via the mechanical properties of the vulcanized, reinforced mixtures prepared therefrom.
Various solutions too have been proposed to overcome the aforesaid disadvantages.
For example, it was proposed in published French patent application No. 71/36250 to granulate the silica in a pan or drum-type granulator, with water as the binder, and to carry out the granulation under agitation, at a pH of less than 6.3.
The disadvantage of such a proposal is the very heterogeneous form of the granulated materials (very broad range of granulometry, high degree of attrition), the difficulties naturally associated with such long drying times (8 to 24 hours) and the need for highly viscous liquids (silica cake referred to as a "suspension" in the patent application) to be homogeneously mixed with powders in a preliminary stage (requiring the use either of very sophisticated, continuous mixers, or very simple but discontinuous mixers of kneading type).
And in U.S. Pat. Nos. 3,646,183 and 3,787,221 it is proposed to granulate precipitated silica, utilizing a surfactant as the binder.
Unfortunately, the presence of the surfactant complicates the drying procedure, i.e., possible decomposition of the binder, and the need for low temperature drying operations with their attendant low thermal yields.
According to U.S. Pat. No. 3,902,915 the problem of granulation is particularly acute when utilizing a rotating drum dryer. This provides too compact a granulate which will not adequately disperse in a composition destined for the making of rubber.
This patent, accordingly, features use of a fluidized bed, but the slurry resulting from the filter cake has a low content in dry solids.
Another method envisaged for the aforesaid drying operation is predicated upon utilizing atomization techniques. On the one hand, however, this provides a finely divided silica, but on the other hand, it is art accepted that a precipitated silica relatively rich in dry solids content cannot be atomized except when working with an acid pH of at most 4.
But in this case the material has to be post-neutralized, as described in French Patent No. 2,230,645; this makes the process complex and markedly increases the cost of the operation.
It too is known that the silica powders can either be compressed or compacted in an attempt to circumvent the aforenoted drawbacks. In such case, however, the products obtained do not flow easily, have low resistance to attrition because of the presence of sharp edges, and do not disperse properly because of their high degree of compactness.
Furthermore, in carrying out such processes, large quantities of material have to be recycled, which of course makes the process considerably more difficult.
Finally, French Patent No. 2,249,834 describes a process which is carried out during the formation of the beads or particulates and which comprises adding to the silica or silicate 5 to 60% by weight of diluent oil emulsified with the water employed to form the beads.
But this process is awkward and inflexible (since it is inextricably tied to the formulation of rubber). It in fact also requires two drying operations, one performed upon the silica itself and the other upon the silica plus the oil-in-water emulsion.
And as for the known method which comprises heating a composition based on grains of silica hydrogel for less than 5 seconds, at from 300 to 1000.degree. C., as described in French Patent No. 2,332,234, this unfortunately also does not provide a product capable of meeting all requirements for use as a reinforcing filler in elastomeric matrices (too compact, 40 to 70 bars pressure, and high cost).
Thus, serious need continues to exist in this art for siliceous pigments and reinforcing fillers which avoid all of those noted, and notable, drawbacks and disadvantages above outlined.